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Old 20th Jan 2020, 8:59 am   #345
Join Date: Apr 2019
Location: Skipton, North Yorkshire, UK.
Posts: 252
Default Re: Gemini 80-Bus System

Whilst investigating the Gotek issue, I have started to get confused over the disk layouts. I would appreciate it if you confirm my understanding. i.e.
  • The Gemini Bios interprets GEMDDDS (double sided 35 track disks) as 70 track disks with 10 (512 byte) sectors per track.
  • The Gemini Bios interprets GEMQDDS (double sided 80 track disks) as 80 track disks with 20 (512 byte) sectors per track.
Is this correct?

With GEMDDDS the disk was accessed in the order; Side 0, Trk 0, Sect 0-9 followed by Side 1 Trk 0 Sect 0-9 etc. and the Goteks 'interleaved' file-layout setting seemed to work well...

#   interleaved:   Interleaved cylinder ordering: c0s0, c0s1, c1s0, c1s1, ...
My confusion is that I cannot get my head around what the physical layout of the GEMQDDS disk is. Looking at the disk image it looks like the sectors are arranged differently with what is presumably CPM stored on sectors 1 to 10 and 21-30 with 11-20 all set to E5. I am wondering if for the purposes of the Gotek whether these need to be shuffled in someway, I certainly can give it a try as it will only take a few lines of code.

In terms of the Gotek, my thinking is that a physical disk has 2 sides, 35/80 cylinders and 10, 512 byte, sectors per track irrespective of how the data is arranged, however, there is a question of how the Gotek accesses the disk image file. Based on the successful GEMDDDS images on the rack system I am assuming it simply accesses them in order.

For completeness, the full range of file layout options are as follows...

# Optional: Image file track layout. Default is "interleaved".
# Comma-separated values:
#   sequential:    Sequential cylinder ordering: all side 0, then side 1.
#   interleaved:   Interleaved cylinder ordering: c0s0, c0s1, c1s0, c1s1, ...
#   reverse-sideN: Side-N cylinders are ordered from high to low (N=0,1).
#   sides-swapped: Sides 0 and 1 ordering is swapped in the image file.
#   eg. "sequential,reverse-side1"
#file-layout = interleaved
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